This invention relates generally to gas turbine engine thrust scheduling, and more particularly to systems and methods for smoothing thrust inputs to gas turbine engines.
One or more throttle levers, such as those found in the cockpit of gas turbine engine powered aircraft, may be used by an operator, such as a pilot, to command one or more turbine engines to a desired thrust to achieve a particular system or aircraft performance, such as maximum cruise. This thrust may be obtained by placing the throttle, either manually or via computer controlled actuation, in a predetermined position, such as the maximum cruise position.
To reduce pilot workload, auto-throttle systems have been developed to electronically control the throttle lever position for a gas turbine engine. Such systems may involve the use of one or more computers, software, memory, and actuators, such as torque motors, to automatically move the throttle lever for the pilot to a particular throttle lever position. Such systems may provide “coarse” and “fine” throttle lever movements for the pilot. For situations where “fine” throttle movements are required, rather than physically moving the throttle lever, the auto-throttle system may send a “TLA fine trim” signal to an engine controller, such as a Full Authority Digital Engine Controller (FADEC), which may then add the value of the “TLA fine trim” signal to a measured throttle lever angle (TLA) to achieve the desired setting.
On most auto-throttle equipped aircraft, however, when the auto-throttle system disengages (whether manually by the pilot or automatically as a result of a pre-programmed event), the “TLA fine trim” setting is automatically set immediately to zero by the engine control system, which may cause a sudden step-change in thrust (i.e., “bump”), which, in turn, may cause a change to a different thrust mode (i.e., from cruise thrust to climb thrust) according to a preprogrammed thrust schedule. To compensate, the pilot may be required to make a manual throttle lever adjustment to return the actual thrust of the engine to that which existed when the auto-throttle system was engaged. Thus, a system and method that automatically eliminates the need for an immediate manual adjustment of the thrust of a gas turbine engine upon disengagement of an auto-throttle system is needed.
In addition, during manual operation of a gas turbine engine, the sensitivity of the throttle lever/engine combination is such that it may be difficult for a pilot to position the throttle lever at a particular TLA to achieve a particular thrust, such as the maximum cruise thrust position or the maximum climb thrust position. As a result, engine component life may be negatively impacted if the TLA is consistently positioned above the desired throttle position because doing so may cause unnecessary wear on the engine due to, for example, higher exhaust gas temperatures and engine rotational speeds. On the other hand, if the TLA is consistently below the desired throttle position, aircraft performance may be less than expected.
To compensate for these occurrences, the TLA-thrust relationship may include “flats” at various thrust levels, as shown in FIG. 1, to permit a pilot to more easily obtain a desired throttle setting during manual operation of a gas turbine engine. The “flats” effectively provide the pilot with relatively wide, angular latitude at specific thrust points to position the throttle to achieve a desired thrust.
However, some auto-throttle systems cannot accommodate “flats” in the TLA-thrust relationship at all thrust points. One example of an auto-throttle system that does not include “flats” at all thrust points in shown in FIG. 2, where the throttle lever angle varies with changes in thrust according to the slope of the line at each point in the TLA-thrust curve between idle thrust and maximum takeoff thrust. Consequently, when operating a gas turbine engine in manual mode (i.e., with auto-throttle disengaged), a system and method is needed that permits a pilot to easily achieve a particular throttle setting on auto-throttle-equipped aircraft that cannot tolerate “flats” in the TLA-thrust relationship. In addition, to continue to operate a gas turbine engine manually without incurring sudden step-changes or “bumps” in thrust during subsequent movements of the throttle lever, the “TLA fine trim” setting may be gradually removed according to the teachings of one or more embodiments of the present invention.